1. Field of the Invention
The invention relates to electrets, i.e. to dielectrics that are permanently electrically polarized because they carry permanent electrical charge.
More particularly, it relates to those electrets that are in the form of sheets or thin layers and usable, in particular, for building electromechanical transducers (microphones, accelerometers, pressure sensors, etc.) or electrostatic relays.
The invention relates in particular to devices for manufacturing electrets of the kind in question.
2. Description of the Prior Art
With known methods of manufacturing such electrets, it is very difficult to obtain uniform distribution of the electric charge carried: charge density is much lower in the vicinity of the edges of the electret than in the center, and may even be zero.
Such lack of uniformity gives rise to various drawbacks, in particular with respect to the sensitivity and/or the stability of transducers or relays built on the basis of the corresponding electrets.
To remedy that drawback, i.e. to obtain uniform distribution of electric charge over the entire charged face of the electret, including right up to its edges, proposals have already been made to place a diaphragm a short distance from said face while it is being charged, the diaphragm carrying electric charge of the same sign as the charge being implanted, and the opening in the diaphragm corresponding to the area to be charged.
That method requires relatively exact positioning in all directions between the diaphragm and the face to be charged, and in the implementation of said method as described in an article by Heinz Von Seggern, published on pages 1623 to 1626 of IEEE Transactions on Industry Applications, Vol. 1A-20 Nov. 1984, New York, the area to be charged was single and separated from the diaphragm by a distance of about 1 millimeter and had a surface area of much more than 10 cm.sup.2.